The present invention relates to a magnetically actuated motion control device. In particular the present invention relates to magnetically actuated motion control devices that vary the contact force between a first member and a second member in accordance with a generated magnetic field.
Magnetically actuated motion control devices such as magnetically controlled dampers or struts provide motion control, e.g., damping that is controlled by the magnitude of an applied magnetic field. Much of the work in the area of magnetically controlled dampers has focused on either electrorheological (ER) or magnetorheological (MR) dampers. The principle underlying both of these types of damping devices is that particular fluids change viscosity in proportion to an applied electric or magnetic field. Thus, the damping force achievable with the fluid can be controlled by controlling the applied field. Examples of ER and MR dampers are discussed in U.S. Pat. Nos. 5,018,606 and 5,284,330, respectively, assigned to Lord Corporation of Cary, N.C.
Generally, MR fluids have high yield strengths and viscosities, and therefore are capable of generating greater damping forces than ER fluids. In addition, the viscosities of MR fluids are precisely controlled by easily produced magnetic fields that are generated by energizing simple low voltage electromagnetic coils. As a result, dampers employing MR fluids have become preferred over ER dampers.
Because ER and MR fluid dampers involve fluid damping, the dampers must be manufactured with precise valving and seals. In particular, such dampers typically require a dynamic seal and a compliant containment member and as a result, prior art MR and ER dampers are not easy to manufacture or assemble. Further, the ER and MR fluid dampers can have significant xe2x80x9coff-statexe2x80x9d forces when the devices are operated at high speeds and the off-state forces can further complicate their manufacture and assembly. Off-state forces refer to those forces at work in the damper when the damper is not energized.
As a result of the shortcomings associated with prior art MR and ER fluid devices, magnetically actuated alternatives to traditional MR fluid motion control devices have been developed. Such magnetically actuated prior art devices are disclosed in pending U.S. Pat. No. 6,378,671 for xe2x80x9cMagnetically Actuated Motion Control Devicexe2x80x9d and in pending divisional application of the allowed ""365 application having Ser. No. 10/080,293, filed Feb. 20, 2002 for a xe2x80x9cSystem Comprising Magnetically actuated Motion Control Devicexe2x80x9d. Both of the issued patent and pending application are assigned to Lord Corporation of Cary, N.C. The prior art magnetically actuated devices disclosed in the applications contain no MR or ER fluid, yet provide a variable level of coulombic or friction damping that is controlled by the magnitude of the applied magnetic or electric field. Prior art magnetically actuated motion control devices overcome a number of the shortcomings associated with MR and ER fluid devices. For example, prior art magnetically actuated motion control devices: may be manufactured and assembled relatively simply and at a relatively low cost; allow for very loose mechanical tolerances and fit between components; do not require a dynamic seal or a compliant containment member; have particularly low off-state forces and provide for a wide dynamic range between the off-state and a maximum damping force. The wide dynamic range is particularly evident when the devices are operated at high speeds.
An exemplary prior art magnetically actuated motion control device disclosed in the pending applications referred to in paragraph [0005] hereinabove is illustrated generally in FIGS. 1, 2 and 3. The prior art motion control device or damper is identified generally at 101 in FIG. 1 and includes a tubular housing 103 defining a cavity 105 in which a piston 107 is located and moveable linearly therein along axis 123. Each end of the damper preferably includes a conventional, well known structure which facilitates attaching damper 101 to other structures, such as clevis eye 121 for attaching the end to a portion of a damped component. The housing 103 includes a least one axially aligned slot 109. The slot may also be referred to as a longitudinally extending slot. The prior art device 101 of FIG. 1 comprises eight slots. All eight of the slots are illustrated in FIG. 2 and five of the slots are illustrated in FIG. 1. The slots pass through the housing wall to define flexible bands, tabs, or fingers 111. The slots 109 extend through the wall of the housing 103 and extend axially nearly the entire length of the housing.
Piston 107 includes a shaft 112 having a magnetically active portion 113 made up of at least one, and preferably two electromagnetic coils 115 set in a magnetically permeable core 117. The portion 113 may also be referred to as a piston head hereinafter. Although here the magnetically permeable core 117 is hollow, the core can alternatively be a solid bobbin. A hollow core allows space for locating connecting wires 119 therein. As shown in FIG. 3, the piston head 113 also defines a plurality of annular poles 114A, 114B, 114C and 114D located adjacent the axially directed portions of the coils 115. The poles 114A-114D have substantially the same dimensions. The poles comprise substantially the same overall axial dimension identified as P in FIG. 3 and a constant lateral dimension equal to approximately one quarter of the diameter D and such lateral dimension is identified as D/4 in FIG. 3. The axial pole dimension P remains substantially constant as the pole extends laterally along the dimension D/4. The poles have substantially rectangular cross sections and hold a constant radial clearance 127 between the outer periphery of the poles and the housing wall when the coils are not energized. In the prior art device 101, the magnetic flux produced when the electromagnets are energized is substantially constant through the poles 114A-D, the inner portion of piston head 113 and housing wall 104, and the constant flux is illustrated by the equally spaced flux lines 125 in FIG. 3. The constant flux is primarily a result of the substantially constant dimensions of the poles, active portion 113 and wall 104.
A current source 118 supplies current to the coils 115 through wires 119. Current flowing through the coils 115 creates a magnetic field that draws the housing 103 in toward the piston head 113. As indicated above, the created magnetic field is illustrated in FIG. 3 by field lines 125. Also shown in FIG. 3 the field surrounds the coil 115 and passes through the poles 114, inner portion of head 113 and housing wall 104. Like head 113, the housing 103 is also made from a magnetically permeable material that will be attracted by the magnetic field including, but not limited to, steels and other iron alloys. The amount of current flowing through the coils 115 is generally directly proportional to the magnitude of the magnetic field generated. Thus, control of the electric current flowing through the coils 115 can be used to control the normal or pressing force between the inner surface of the housing 103 and the outer surface of the piston 107, thereby controlling the damping effect of the damper 101.
The slotted housing 103 and the head 113 of the piston 107 are preferably made from low carbon, high permeability steel, although other magnetically permeable materials can be used. The slots 109 are preferably evenly spaced around the circumference of the housing 103 so that axial-periodic symmetry is maintained. The pair of coils 115 is preferably wired such that they produce magnetic fields in opposite directions as indicated by the directional arrows associated with the field lines 125 illustrated in FIG. 3. This configuration allows the magnetic field produced by each coil 115 to add rather than cancel in an area between the adjacent coils 115.
An illustration of the damping effect produced by device 101 can be seen in the lateral sectional view shown in FIG. 2, which shows the relationship of the slotted housing 103 with respect to the piston 107. When no magnetic field is applied, the piston 107, and particularly the head 113, fits loosely within the housing 103 to define a small radial clearance 127 between the housing 103 and the magnetically active portion 113 of the piston 107. That is, the housing 103 is relaxed and does not press against the piston head 113. When current is supplied to the coils 115 the magnetic field generated causes the flexible fingers 111 in the housing 103 to be attracted radially inward as indicated by the arrows 126 such that the housing 103 squeezes the piston 107 with a force proportional to the applied magnetic field, and therefore the applied current.
Although prior art damping devices are an effective source of damping in many applications, there are shortcomings associated with the prior art magnetically actuated device 101. The hollow configuration of the active portion 113 of piston 107 and the properties of the magnetically permeable materials comprising the poles 114 and head 113, cause the devices to become magnetically saturated. As a result of such saturation, prior art devices are limited in the magnitude and range of damping forces that may be provided. Prior art magnetically actuated devices do not maximize the magnetic flux at the area of contact between the housing and piston. Rather prior art devices provide for substantially the same magnitude magnetic flux at and away from the area of contact between the housing and piston head when the magnetic field is generated.
The foregoing illustrates limitations known to exist in present devices and methods. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.
According to one aspect of the invention, a magnetically actuated motion control device is provided. The magnetically actuated motion control device includes a a first member defining a cavity; a second member positionable within the cavity and being movable relative to the first member along an axis when positioned therein, the second member comprising at least one pole, the at least one pole having a first portion comprising a first axial dimension and a second portion having a second axial dimension, the first axial dimension being greater than the second axial dimension; at least one of the first member and the second member including at least one moveable finger; a magnetic field generator located on another of the first member and the second member, the magnetic field generator causing one of a portion of the first member and a portion of the second member to press against the other of the portion of the first member and the portion of the second member.
By decreasing the axial dimensions between an inner first pole portion and an outer second pole portion the magnetic flux, xcfx86, per unit area, also referred to as flux density, xcex2, is increased at the outer contact surface comprising the second pole portion. In this way the poles serve to channel or funnel the magnetic flux between the inner and outer portions of the poles. The flux density may be represented by the following equation: xcex2=xcfx86/Area. The magnetically actuated devices of the present invention provide a greater range of dynamic damping forces than conventional piston devices.
The poles may comprise any suitable cross section including but not limited to a wedge-shaped cross section or a cross section that has a substantially rectangular portion with a contact portion extending outwardly from the substantially rectangular portion. The movable member may comprise any suitable number of piston heads. Each piston head may comprise any number of poles and the poles may be substantially the same or different.
The foregoing and other aspects will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawing figures.